Heretofore, electrolytic electrodes comprising a substrate of valve metals, e.g., titanium (Ti), have been used as superior insoluble metal electrodes in the field of electrochemistry. In particular, they have been widely used as anodes for the generation of chlorine in the salt (sodium chloride) electrolytic industry. In addition to Ti, tantalum (Ta), niobium (Nb), zirconium (Zr), hafnium (Hf), vanadium (V), molybdenum (Mo), tungsten (W), etc. are known as valve metals.
These metal electrodes are produced by coating metallic titanium with various electrochemically active substances such as platinum group metals and their oxides. Examples of such platinum group metals and their oxides are described in, e.g., U.S. Pat. Nos. 3,632,498 and 3,711,385. As electrodes for the generation of chlorine, these electrodes can maintain a low chlorine overvoltage over a long period of time.
However, when the above metal electrodes are used as anodes in electrolysis for the generation of oxygen or electrolysis in which the generation of oxygen is involved, the anode overvoltage gradually increases. In extreme cases, the anode is passivated and thus it becomes impossible to continue the electrolysis.
The phenomenon of passivation of the anode is believed to be caused mainly by the formation of electrically non-conductive titanium oxides that result from (1) the oxidation of the titanium base material with oxygen by the electrode coating-constituting oxide substances itself; (2) oxygen diffusion-permeating through the electrode coating; or (3) an electrolyte.
Formation of such electrically non-conductive oxides in the interface between the base material and the electrode coating causes the electrode coating to peel off. This creates problems such as the breakdown of the electrode.
Electrolytic processes in which the anode product is oxygen, or oxygen is generated at the anode as a side reaction, include: (1) electrolysis using a sulfuric acid bath, a nitric acid bath, an alkali bath or the like; (2) electrolytic separation of chromium (Cr), copper (Cu), zinc (Zn), or the like; (3) various types of electroplating; (4) electrolysis of dilute salt water, sea water, hydrochloric acid, or the like; and (5) electrolysis for the production of chlorate, and so forth. These processes are all industrially important. However, the above-described problems have hindered the metal electrodes from being used in these processes.
U.S. Pat. No. 3,775,284 has disclosed a technique to overcome passivation of the electrode due to permeation of oxygen. In this technique, a barrier layer of a platinum (Pt)-iridium (Ir) alloy, or oxides of cobalt (Co), manganese (Mn), lead (Pb), palladium (Pd), and Pt is provided between the electrically-conductive substrate and the electrode coating.
The substances constituting the intermediate barrier layer can prevent the diffusion-permeation of oxygen during electrolysis to some extent. However, these substances are electrochemically very active and therefore, react with an electrolyte coming through the electrode coating. This produces electrolytic products, e.g., gas, on the surface of the intermediate barrier layer which gives rise to additional problems. For example, the adhesion of the electrode coating is deteriorated under the physical and chemical influences of the electrolytic products. Thus, there is the danger of the electrode coating peeling off before the life of the substance constituting the electrode coating has expired. Another problem is that the corrosion resistance of the resulting electrodes is poor. Thus, the method proposed in U.S. Pat. No. 3,775,284 fails to produce electrolytic electrodes which are of high durability.
U.S. Pat. No. 3,773,555 discloses an electrode in which a layer of an oxide of, e.g., Ti, and a layer of a platinum group metal or its oxide are laminated and coated on the electrode. However, this electrode suffers from the problem that when it is used in electrolysis in which the generation of oxygen is involved, passivation occurs.